1. Technical Field of the Invention
The present invention relates to a digital watermarking apparatus and a computer-readable medium for embedding a data in an index color image data.
2. Background of the Related Art
Recently, digital watermarking is used for the sake of giving copyright protection to an image, or the like. Practically, contents on which digital watermarking is to be applied are restricted to those in which the number of colors is large, such as full-color ones. There has been no technique for embedding a data in an image which can be expressed only by a relative small number of colors or 256 or less colors (for example, an 8-bit index color image). This is caused because only full-color images have been considered as contents which are to be subjected to copyright protection. It has been considered that digital watermarking is not needed in an image which can be expressed only by 256 or less colors. Moreover, there is a technical problem in that, in an image which can be expressed only by 256 or less colors, the range where the pixel color can be changed is restricted and hence digital watermarking is not suitable for information hiding.
A conventional example of digital watermarking is disclosed in JP-A-11-164129. The reference discloses a technique in which a data is embedded in a frequency region of an image data.
However, there is a need of embedding information in an image which can be expressed only by 256 or less colors. For example, a system will be considered in which an image data is uploaded from a cell phone to a server, and the image data is downloaded by another cell phone, thereby enabling the image data to be shared by plural persons. In such a system, it may be requested to know the number of situations where the image data which has been once downloaded is again uploaded to the server. In this case, the number of uploading operations may be embedded in the image data. If all cell phones can display an image of 256 or more colors, there arises no problem. In the case of a cell phone which can display only an image of 256 or less colors, such as GIF, it is impossible to check an image data which is uploaded from the cell phone. The above-mentioned need exists in such a system. The invention has been conducted in view of the above-discussed circumstances.